The explanation will use an example of an x-ray imaging device as the radiographic device.
In an x-ray imaging device, x-rays are emitted from an x-ray tube towards an object to be examined, and those x-rays that pass through the object to be examined are detected, and x-ray imaging is performed by producing an x-ray image based on the detected x-rays. When the x-rays pass through the object being examined, scattered rays are produced through collisions of the x-rays with the object being examined, and blurring of the image results from the scattered rays.
Given this, in order to eliminate the scattered rays, which have been scattered by the object being examined, in x-ray imaging devices a grid is used wherein substances having high x-ray absorbency and substances having low x-ray absorbency are lined up in parallel alternating at fixed intervals, where a moire pattern is produced in the image that is produced by the x-ray detecting device due to the differences in the spatial frequency possessed by the grid (the array spacing) and the spatial sampling period (the detection pixel spacing) of the x-ray detecting device.
Because of this, conventionally a variety of measures have been put in place in order to eliminate or reduce the moire pattern in this type of imaging:
(1) The use of a grid having the same frequency characteristics as the x-ray detecting device, so as to not produce the moire pattern is one of these known measures;
(2) There is also the cancellation of the moire pattern through moving the grid during x-ray exposure;
(3) Along with this, in order to eliminate the high-frequency components such as the moire pattern (which may also be termed the “grid pattern”) that comprises a grid frequency component (shown in FIG. 5(e)), such as appears in an x-ray emission image (=“original image,” such as shown in FIG. 6) when using a grid, conventionally [A] the application of low-pass filtering (LPF) in a direction that cuts across the grid pattern perpendicularly (that is, in the direction that is perpendicular to the grid elements) has been performed, or [B] after extracting high-frequency components that include the grid frequency component, by performing high-pass filtering (HPF) in a direction that cuts across the grid pattern perpendicularly, the high-frequency components are subtracted from the original image, to eliminate the grid high-frequency components, or the like, are known as examples of such processes.
Furthermore, [C] a grid image (a band-pass grid image) is produced by performing band-pass extraction of the grid high-frequency components from an original image by passing it through a band-pass filter (BPF) (as shown in FIG. 5(a)). An image processing technology has also been proposed wherein the band-pass grid image is subtracted from the original image in order to exclude the grid-frequency components from the original image.
Describing this in specifics, when a Fourier transform is performed on the original image to calculate the spectrum intensities, if, as illustrated in FIG. 12, the binning size in the horizontal direction is x1 pixels, then the peaks of the grid frequency will appear in the vicinity of ½x and 1x the Nyquist frequency Nq (as illustrated in FIG. 13(a)). Similarly, in the case of an image where in the binning size in the horizontal direction is x2, then a grid frequency peak will appear in the vicinity of 1x the Nyquist frequency Nq (as shown in FIG. 13(b)).
When it comes to the frequencies at which the peaks occur, if the array spacing of the grid is defined as 1g and the detecting element spacing thereof is 1p, then if 1g<1p, the moire pattern pitch 1m will be:1m=1p×1g/(1p−n×1g).
Here n=0, 1, 2, . . . , and if 1g is no more than 2×1p then n will be 1, but if greater than 2x and no more than 3x, then n will be 2, and thus the frequency fm corresponding to the grid frequency peak will produced as fm=(1p−n×1g)/1p×1g (where the second harmonic is twice this).
Note that the peak on the right side, i.e., the second peak, shown in FIG. 5(a) corresponds to the second harmonic of the peak of the grid frequency on the left side (where the frequency where the peak appears=fm).
The band-pass extraction described above is a technique for detecting these peaks to extract the applicable frequencies from the original image. However, there still remains the problem of residual moire pattern, as described below, that exists in frequency domains other than the peaks due to variability in the grid, and the like.
When it comes to this, as can be understood from FIG. 8, which relates to the example of embodiment set forth below, and from the explanation thereof, vertical lines will remain throughout FIG. 8. As illustrated in FIG. 8, under the conventional technology it is only possible to eliminate the grid frequency component to this degree, where there is residual moire pattern in x-ray illumination images from which the grid images (the band-pass grid images) have been subtracted.
Further, an example of this can be found in Japanese Unexamined Patent Application Publication 2002-325755.
Here, in the case (1) wherein a grid having the same frequency characteristics as the x-ray detecting device is used, this should not be effective unless the matching is strict, and thus extremely high levels of positional accuracy and manufacturing accuracy for the grids are required, which cannot be produced inexpensively. For example, even if such grids could be produced, if there were a variation in the SID (the distance between the x-ray source and the x-ray detecting device), then the match between the frequency characteristics would be disrupted, where even a minute variation in the SID will produce a frequency difference, making the moire pattern visible.
Moreover, in the case (2) wherein the grid is moved during the x-ray emission, a separate moving mechanism or device is required, and thus there are problems in that this not only leads to an increase in overall size of the equipment, but also leads to increases in manufacturing costs.
Furthermore, when (3) image processing is performed so as to eliminate the high-frequency components that include also the grid frequency, the pattern that is produced in the original image by the grid cannot be extracted and cut out by only the [C] band-pass, for example, and thus there will be the problem of a residual pattern in the original image from which the grid image has been subtracted.
Given this, in contemplation of the above, the object of the present invention is to provide an improved x-ray imaging device able to resolve the residual moire pattern that is due to differences between the grid spatial frequency characteristics and the x-ray detecting device spatial frequency characteristics, able to do so through image processing through an extremely simple structure, without problems in terms of cost.